OPERATION
3.7.2 Guarded Amps and Coulombs
NOTE
The following discussion on guarded amps also
applies to the Coulombs function.
The unguarded and guarded output configurations for
amps is shown in Figure 3-13. With guard enabled, out-
put low is disconnected from the inner shieldof the OUT
PUT connector, and the internal guard drive is connected
to that inner shield. Output low can be routed to the outer
shell of the output connector b installing the shorting
link. However, installing the l&connects output low to
chassis ground, eliminating the floating capability of the
instrument.
There is no need to use p”d if the Model 263 is sours-
mg current to a feedbac plcoammeter as ths cucmt IS
already guarded. Figure 3-14 shows the Model 263 con-
nected to a feedback picoammeter. Because the voltage
drop (burden voltage) axoss the input of the picoammeter
is very low (typically <lmV), the voltage on HI and LO
are are virtually identical. With the same voltage poten-
tial, no leakage current can flow.
The Model 263 can be used in the guarded configuration
if it is connected to a shunt picoammeter. The shunt or
ation is onl
available on older elec-
i%ezc?E%Keithley h&dels 602 and 610C. The
input of a shunt picoammeter is actually configured as a
shunt electrometer voltmeter. The guarding technique is
similar to the one used for sourcing ohms to an electro-
meter using an input adapter. Both the Keithley Models
6191 and 6167 Input Adapters can be used with the Model
602. Figure 3-15 shows how the Model 263 is connected
to a shunt picoammeter (Model 602 set for ‘NORMAL”
amps) using an input adapter (Model 6191). Notice that
the guard cable from the input adapter is not connected
to the electrometer since guard voltage is provided by the
Model 263.
Guarding is recommended when sourcing current or
charge to ahigh impedance load. Arecommended guard-
ing method is shown in Fi
8”
e 3-16. With this method out-
put high is guarded all t
e way to the load.
In AMPS and COWL, guard drive is also available at the
PREAMP OLJT terminal on the rear panel. Guard drive
is available at that terminal in both the guarded and
unguarded configurations. Figure 3-17 shows an applica-
tion for the external guard. This configuration makes it
possible to source current to a particular resistor (Rl)
without disconnecting other components (R2 and R3) in
the resistor network. As configured, all of the output cur-
rent will flow through Rl. This is because virtually the
same voltage potential exists on both sides of RZ. In reality,
a slight voltage difference exists because of the voltage off-
set of the preamplifier. In most situations, the resultant
current through R2 is insignificant. The current that flows
from PREAMP OUT through R3 is of no consequence.
In AMPS V/R and COLJL V/R, guard drive is not available
at I’REAMP OUT. Since AMPS V/R and COUL V/R are
usually used to calibrate electrometers, grounding is not
needed. If sourcing AMPS V/R or COUL V/R to a load,
a guard drive will have to be obtained from an external
device.
3-15
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